The invention relates to the allocation of channels in a TDMA communications system with multiple earth stations and, more particularly, to an apparatus and method for allocating channel capacity among the stations to equalize the probability of call blocking across a network, and to minimize the call blocking probability while maintaining a moderate average reallocation frequency.
Time division multiple access systems that utilize a satellite to provide a communications link between a plurality of earth stations are necessarily limited to a particular number of system channels.
In a TDMA system, each channel corresponds to a fixed number of sequentially-transmitted bits of information. The total number of available channels is determined by the transmitting bit rate, which in turn is limited by the transponder bandwidth and power of the satellite.
Each channel provides a single voice or data communications path between a transmitting and a receiving ground station. The transmitting station transmits its signal to a satellite transponder, which retransmits the signal to the receiving station. For so long as data or voice signals are transmitted over a communication path, the associated channel is in use and is therefore not available to establish a further communications link. Blocking results when an attempted call can not be completed due to the unavailability of an idle communications channel.
Typically, each TDMA earth station periodically transmits a multi-channel burst with a burst duration that is a function of the number of transmitted channels. The transmission bursts of the stations occur in a sequential non-overlapping timed sequence and a fixed duration system transmission cycle or frame includes the transmission bursts of all of the stations. Thus, an increase in channel capacity for a particular station necessarily requires an increased burst duration for the station and, since a TDMA frame is of fixed duration, an increase in burst duration or channel capacity for a particular station must necessarily arise from an allocation of extra channels from a pool of unused frame channel capacity or a corresponding reduction in the channel capacity or burst duration of another station. Therefore, since the total number of communication paths is limited by the number of available channels, it is apparent that some scheme must be employed to assign or dedicate particular channels to specific stations so that each station is assured that at least a few communications channels will be available at any given time.
One such prior art allocation scheme permanently assigned a fixed number of channels to each station, based upon the station's average expected communications traffic. However, as indicated in the U.S. Pat. No. 3,644,678 to Schmidt, "Channel Reallocation System and Method", such a permanent preassigned channel allocation system is inefficient due to the fact that large fluctuations in the traffic load at a particular station results in excessive blocking of signals during heavy load periods and inefficient use of channel capacity during light load periods.
Thus, in order to avoid such inefficiency, it is necessary to allocate system channels among the various ground stations on a demand basis. In accordance with a system of this type, each ground station periodically demands a number of communication channels that is sufficient to handle its current traffic load. A reference station in the communication network thereafter allocates to each station the number of channels necessary to meet its demand and then allocates the excess system capacity equally among the stations.
It has also been proposed to allocate excess channel capacity in proportion to station capacity requests in a demand allocation system. However, this type of allocating scheme results in unequal blocking probabilities at different stations since an earth station that requests a small amount of capacity will necessarily experience a higher proportion of blocked calls than earth stations in the same network that request more capacity. Thus, such a demand allocation system would necessarily require a high reallocation rate in order to reduce the probability of blocking.
A demand allocation system as described in the U.S. Pat. No. 3,644,678, would allocate excess capacity equally to each earth station in the network. However, although this method would simplify the reallocation computations, it would also result in unequal blocking probabilities at the different earth stations, since stations requesting large amounts of capacity would necessarily have a higher proportion of blocked calls than stations requesting less capacity.
Accordingly, it is an object of the invention to provide a simple and effective apparatus and method to reallocate excess system capacity in order to maintain a uniformly low blocking probability at earth stations that have widely varying capacity requirements.
A further object of this invention is to provide a simple and effective means to reduce the reallocation frequency of the system and to thereby reduce the computational time expended in operating the system.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.